1. Field of the Invention
This invention relates to a double axis type transit which is capable of effecting repeated measurement of an angle to be measured.
2. Description of the Prior Art
The repetitive measuring method is effected to reduce the influence of the error resulting from the structural defect of the transit or the incompleteness of adjustment as well as the influence of the error resulting from the observation operation such as collimation or reading. The transit is comprised of a base, first and second rotational members coaxial with the base and rotatable, means for measuring the angle between the first rotational member and the second rotational member, and a collimator secured to the first rotational member. Accordingly, the repetitive measuring method, as shown in FIG. 1 of the accompanying drawings, comprises installing the transit at a point 0, fixing the base thereof, and rotating the first rotational member with the second rotational member to thereby collimate a point A by the collimator. Let it be assumed that the then angle of the first rotational member with respect to the base is .alpha..sub.0. Subsequently, the first rotational member is made rotatable with the second rotational member remaining fixed to the base and a point B is collimated by the collimator to thereby measure an angle .alpha..sub.1. Then, the first rotational member and the second rotational member are rotated together relative to the base and the point A is again collimated by the collimator, and subsequently the second rotational member is fixed to the base, while the first rotational member is made rotatable and the point B is collimated by the collimator to thereby measure the angle. The cumulative measured value when the space between the points A and B has been measured n times in this manner is defined as .alpha..sub.n, and the angle .alpha. of &lt;AOB is found from EQU .alpha.=(.alpha..sub.n -.alpha..sub.0)/n
Now, the transit disclosed in Japanese Laid-open Patent Application No. 165011/1983 comprises a base, a first rotational member having a collimator and supported on said base for rotation about a vertical axis, a second rotational member supported for rotation about said vertical axis relative to the base and the first rotational member, measuring means for measuring the relative angular displacement between said first rotational member and said second rotational member by a binary code, digital display means for displaying the measurement value of said measuring means, digital display clearing means for rendering the digital display means into a zero value when the measurement value of said measuring means has become 360.degree., and inhibiting means for selectively inhibiting the clearing function of said digital display clearing means, and has been designed such that the digital display means displays a right measurement value even when the measured angle by the repetitive measuring method or the like exceeds 360.degree..
However, in the above-described apparatus, the inhibition of the clearing function of the digital display clearing means by the inhibiting means is effected in a circuit-like fashion, and this has led to a disadvantage that the apparatus is complicated in structure.
Also, suppose a case where the maximum countable number of a counter for counting the binary code corresponding to the relative angular displacement is not integer times as great as 360.degree.. When this counter counts the maximum countable number, e.g., 1000, the content of the counter returns to zero and therefore, when the counter counts down from this state, the content of the counter changes to 999, 998, . . . . However, where subtraction is effected from zero (it is to be understood that the angle changes by 1.degree. each), the angle changes to 359, 358, . . . . Accordingly, where the maximum countable number of the counter is not integer times as great as 360.degree., the count number of the counter must be corrected, and this leads to a disadvantage that the apparatus becomes complicated.